Bypassing Fuel Tank Pressure Sensors: Diy Guide And Precautions

The fuel tank pressure sensor is a 3-pin or 3-wire sensor with a 5V source voltage. The output voltage varies from 0.1V to 4.99V depending on the amount of pressure or vacuum. The sensor is used as part of the evap system and is necessary to pass emissions tests. Bypassing the sensor is not recommended as it may cause issues with the evap system and turn on the check engine light. However, it is possible to fool the sensor by creating a voltage divider circuit with two resistors, which provides a fixed output voltage to the ECM, making it think the pressure is constant.

Using fixed resistors to trick the DME into thinking the tank is pressurised

To trick the DME into thinking the tank is pressurised, you can use fixed resistors to create a simple voltage divider. The DME is looking for a 0-5V signal from the sensor. By connecting two 1k ohm resistors and three wires (red, blue, and black) in a specific configuration, you can achieve a +2.5V signal on pin #11, which is the sensor voltage. The red wire taps into pin 11, the blue wire into pin 44, and the black wire into pin 40. This will make the DME think the pressure is present and clear the check engine light.

However, it's important to note that the fuel tank pressure sensor has multiple functions and readings, and simply providing a fixed voltage may not be sufficient in the long run. The sensor also needs to read atmospheric pressure (2.6 volts) when the tank is vented and deep vacuum (1.3 volts) when the PCM performs the evap running loss test. It also needs to change voltages during the evap vent test and vapor generation test. Therefore, while providing a fixed voltage may work temporarily, it is likely to cause issues and trigger error codes during certain driving conditions or when specific tests are performed by the PCM.

Additionally, it is always recommended to consult a qualified mechanic or technician before performing any modifications to your vehicle, especially when dealing with sensitive components and computer systems.

Using a potentiometer to adjust voltage

A fuel tank pressure sensor is an important part of a vehicle's fuel system, functioning as a watchdog to monitor the pressure inside the tank and transmit information to the engine control unit (ECU). While these sensors are crucial for maintaining proper fuel pressure and ensuring the engine runs smoothly, there may be times when you need to bypass or adjust the sensor. This can be done using a potentiometer to adjust the voltage.

Understanding the Sensor and Its Function

The fuel tank pressure sensor is an electronic device with three wires: power, ground, and sense. It measures the pressure of the fuel in the tank and sends a signal to the ECU when the pressure reaches a certain level. The ECU then adjusts the fuel pump accordingly to maintain the ideal fuel pressure.

Reasons for Adjustment

There are several reasons why you might need to adjust the voltage of the fuel tank pressure sensor. One common issue is a faulty sensor that sends incorrect readings to the ECU, leading to problems such as a check engine light, poor fuel efficiency, engine stalls, or misfires. Additionally, wear and tear, moisture damage, vibration, and electrical problems can cause the sensor to fail over time.

Using a Potentiometer for Adjustment

A potentiometer is a variable resistor that can be used to adjust the voltage of the fuel tank pressure sensor. By placing the potentiometer in the signal wire, you can increase or decrease the voltage to trick the ECU into thinking the fuel tank pressure is within the acceptable range. This method allows you to fine-tune the voltage without completely bypassing the sensor.

Step-by-Step Guide

• Locate the fuel tank pressure sensor near the fuel tank or on a nearby component, such as the fuel pump.
• Disconnect the sensor from the wiring harness by unplugging its connector.
• Install a bypass harness, which is a type of wiring harness that allows you to bypass the sensor. Connect it between the sensor and the wiring harness.
• Adjust the potentiometer to change the voltage. You may need to refer to the sensor's voltage chart to determine the correct voltage range.
• Test the system by starting the engine and checking for any warning lights or issues.

Considerations and Precautions

• Bypassing or adjusting the fuel tank pressure sensor can impact your vehicle's performance and efficiency. Consult a mechanic or professional before making any changes.
• Ensure that any adjustments you make fall within the acceptable voltage range for your vehicle's specific sensor.
• Be cautious when working with electrical components, and always disconnect the battery before making any modifications.
• Regularly inspect and maintain your fuel tank pressure sensor to prevent issues and ensure accurate readings.

By following these steps and using a potentiometer to adjust the voltage, you can bypass the fuel tank pressure sensor and fine-tune the fuel pressure readings in your vehicle.

Disconnecting the sensor might erase the DTC but will trigger a check engine light

Disconnecting the fuel tank pressure sensor will likely trigger the check engine light, as this is the only sign of a faulty sensor. However, there are ways to bypass the sensor without triggering the check engine light.

The fuel tank pressure sensor is part of the evaporative emissions control (EVAP) system, which captures and recycles fuel vapors from the fuel tank to prevent them from escaping into the atmosphere. The sensor measures the pressure inside the fuel tank and transmits this information to the engine control module (ECM) or powertrain control module (PCM). The ECM uses this information to determine if the fuel tank is sealed correctly and to monitor the EVAP system for leaks or malfunctions.

If you simply disconnect the sensor, the ECM will detect an issue and trigger the check engine light. Additionally, the EVAP system may fail, leading to increased emissions and potential environmental damage. Therefore, it is essential to have any emissions-related issues diagnosed and repaired by a qualified technician to ensure compliance with local and federal regulations.

However, there are ways to bypass the sensor without triggering the check engine light. One method is to use fixed resistors to trick the ECM into thinking the tank is pressurized. This involves determining the voltage the sensor is looking for and using resistors to mimic this voltage. Another method is to use a simulation circuit, which involves hooking up the sensor wire to an oscilloscope to figure out the signal it is sending to the ECM and then building a circuit to mimic this signal.

It is important to note that bypassing the fuel tank pressure sensor is generally not recommended, as it can affect the performance of the EVAP system and lead to increased emissions. Additionally, it may be illegal in some jurisdictions to bypass the sensor, as it is an integral part of the emissions control system. Therefore, if you choose to bypass the sensor, you should do so with caution and ensure that you are compliant with all relevant regulations.

Shorting the 5-volt reference to the return signal will provide 5 volts to the ECM

Firstly, it is important to understand the function of the 5-volt reference signal. In modern vehicle design, the 5-volt reference signal is a crucial aspect of vehicle command and control. It typically flows through a sensor with a resistance that varies according to changes in temperature, pressure, or position. The signal return voltage to the ECM is always less than the reference voltage due to this variable resistance.

Now, let's discuss the procedure to bypass the fuel tank pressure sensor by shorting the 5-volt reference to the return signal:

• Identify the Wires: The fuel tank pressure sensor typically has three wires: power, ground, and sense. In the case of the BMW mentioned in the source, the wires are grey-white (power), brown-black (ground), and yellow-red (sense). Ensure you have correctly identified these wires before proceeding.
• Determine the Voltage Requirements: Different vehicles may have different voltage requirements for the fuel tank pressure sensor. In the case of the 2005 Mercury Montaineer, the sensor needs to read atmospheric pressure (2.6 volts) when the tank is vented and deep vacuum (1.3 volts) during certain tests. You will need to determine the specific voltage requirements for your vehicle.
• Calculate the Resistor Values: To provide a 5-volt signal to the ECM, you may need to use resistors to manipulate the voltage. In the case of the 1998 Jeep Cherokee Sport mentioned in the source, the technician used two 1k ohm resistors in a voltage divider configuration to provide a 2.5-volt signal to the ECM, clearing the check engine light. Calculate the appropriate resistor values based on your vehicle's voltage requirements.
• Connect the Resistors: Connect the resistors in the calculated configuration to the appropriate wires. Ensure that you make secure and reliable connections to avoid potential issues.
• Test the Setup: Once you have connected the resistors, start the vehicle and use a scan tool to monitor the fuel tank pressure sensor data. Ensure that the ECM is receiving the expected 5-volt signal and that there are no error codes or abnormal behavior.
• Secure the Setup: If the setup is functioning correctly, carefully secure the resistors and connections to prevent any damage or disconnection while the vehicle is in use. Ensure that all components are mounted away from moving parts and sources of excessive heat.

By following these steps, you can bypass the fuel tank pressure sensor by shorting the 5-volt reference to the return signal, providing a 5-volt signal to the ECM. Please note that bypassing sensors may affect the vehicle's performance and emissions, and it is recommended to consult with a qualified technician or mechanic before making any modifications.

Using a voltage divider to build a fixed sensor

Building a voltage divider to bypass the fuel tank pressure sensor is a possible solution, as suggested by some users. However, it is important to note that this method may not work for all vehicles and should be approached with caution.

The pressure sensor typically has three wires: 0V, 5V, and a sensor output voltage ranging from 0-5V. The sensor output voltage is dependent on the pressure in the tank, and this voltage is what the DME (engine control unit) or ECM (engine control module) reads to determine the pressure.

To bypass the sensor, you can create a simple voltage divider using resistors to simulate a specific voltage, tricking the DME/ECM into thinking there is a certain pressure in the tank. In the case of a BMW M Roadster, a user reported success by using two 1k ohm resistors and three wires (red, blue, and black). The red wire taps into pin 11, the blue wire into pin 44, and the black wire into pin 40, resulting in a +2.5V output, which the DME interprets as the correct pressure, thus clearing the check engine light.

It is important to note that this solution may not be permanent, as the DME may be looking for a voltage change rather than a static voltage. Additionally, different vehicles will have different pinouts, so it is crucial to refer to the electrical wiring diagrams for your specific vehicle before attempting this method.

Another user suggested a similar approach for a 1999 Burban with a custom fuel cell, recommending the use of a voltage divider calculator to determine the appropriate resistor values to provide the ECM with a "happy voltage." However, they also noted that finding the right resistor values might take some time due to the various EVAP solenoid checks that could trigger error codes.

It is worth mentioning that bypassing the fuel tank pressure sensor is generally not recommended, as it plays an important role in the vehicle's EVAP (evaporative emission control) system, which prevents fuel vapors from escaping into the atmosphere.

Frequently asked questions